Because I expect the economics of enough batteries to carry a data center’s worth of demand through the night and morning is too expensive compared to essentially free natural gas.
Data centers would ideally run at ~100% utilization, so any drop in solar output needs to be fully met by batteries.
For AI data centers? Depreciation on the GPUs and AI accelerators is a bigger cost than electricity.
Now it might be that a new data center will find it more difficult to obtain a reliable supply of electricity than to obtain GPUs, but your statement was about expenses.
Data centers would ideally run at ~100% utilization, so any drop in solar output needs to be fully met by batteries.